pouchdb: Error 500 with CORS warning

I am getting repetitive 500 errors (v5.1.0) and would appreciate some guidance on how to debug further.

Uncaught (in promise) e
error: true
message: "Database encountered an unknown error"
name: "unknown_error"
result: Object
doc_write_failures: 0
docs_read: 0
docs_written: 0
end_time: Tue Dec 08 2015 21:00:53 GMT+0200 (SAST)
errors: Array[1]...
ok: false
start_time: Tue Dec 08 2015 20:14:09 GMT+0200 (SAST)
status: "aborting"
status: 50

followed by a warning

PouchDB: the remote database may not have CORS enabled.If not please enable CORS: http://pouchdb.com/errors.html#no_access_control_allow_origin_header

but I definitely have cors enabled

curl --user XXX:XXX xxxx.cloudant.com/_api/v2/user/config/cors 
{
  "enable_cors": true, 
  "allow_credentials": true, 
  "origins": [
    "*"
  ]
}

this happens periodically, but especially when offline. I am canceling the sync as follows when offline is detected:

// on startup / online detected:
sync.company = db.sync(companyUrl, {live: true, retry: true})...

// if offline detected
console.log('offline detected - stopping sync');
if (sync.company) sync.company.cancel();

but am also seeing this error

offline detected - stopping sync
TypeError: o.company.cancel is not a function
    at e (vendor.js:62)
    at Object.fn (vendor.js:62)
    at n.$digest (vendor.js:5)
    at n.$apply (vendor.js:5)

(code has been minified)

inspecting the sync.company object i see it is a promise… but the docs show that you should be able to call cancel() ??

http://pouchdb.com/api.html#sync

not sure if the sync cancel is in any way related to the 500 error, but thought it would be worth mentioning